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outside air flow rate does not match cfm specified under systems tab in EnergyPlus

asked 2014-12-02 09:41:39 -0600

tinadarko gravatar image

updated 2014-12-02 10:57:54 -0600

Hello,

I am relatively new to Openstudio/Energyplus and have run into some problems recently:

I am trying to specify the OA flow rate under the systems tab in OpenStudio, but when I run the model the results show that the zone mechanical ventilation current density flow rate does not match what I specify. I have put in cfm numbers for minimum outdoor air flow rate and maximum outdoor air flow rate under the OS:Controller:OutdoorAir object. Is there another control switch that I have to turn on to hard size the OA flow rate?

Thank you in advance.

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answered 2014-12-05 13:59:18 -0600

updated 2015-01-09 13:51:36 -0600

This answer follows from the comments in the answer from Kyle Benne. Note that Kyle said that OpenStudio will "always write the mechanical ventilation controller which is referenced by the oa controller". This is a reference to Controller:MechanicalVentilation which uses the DesignSpecification:OutdoorAir objects for control. So, if I am following correctly, in OpenStudio, the DesignSpecification:OutdoorAir values are used both for autosizing the minimum outdoor air flow rate and for controlling the minimum outdoor air flow rate during the simulation.

Controller:MechanicalVentilation computes a minimum required outdoor air flow at each time step, but that is not the final answer. Other inputs in Controller:OutdoorAir can ask for more OA. So, all of the various OA inputs need to be coordinated.

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You are following correctly. OpenStudio attempts to use the design specification for autosizing and for controlling the simulation. That is not so say folks can't go into the oa controller and have their way.

Kyle Benne gravatar image Kyle Benne  ( 2014-12-05 15:43:05 -0600 )edit

Thanks Michael and Kyle, that definitely cleared things up for me. I appreciate both of you taking the time to answer my questions!

tinadarko gravatar image tinadarko  ( 2014-12-10 22:54:24 -0600 )edit
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answered 2014-12-02 10:00:36 -0600

Autosize is a little different then you might expect for these fields. If the oa rates are autosized, in an OpenStudio style idf, the simulation will look "up" to the design oa specs attached to the zones. I suggest that you leave the oa controller fields autosized and use the design oa spec as outlined in this question.

https://unmethours.com/question/757/h...

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Thanks Kyle, the link was very helpful. So even if I put in a hard sized number in the OA controller fields, it will get overridden by the design OA specs in each space type, correct? If I would like to have OA turned off during unoccupied hours, do I assign the OA schedule to the OS:Controller:OutdoorAir object, or put it under the design OA specs object?

Many thanks!

tinadarko gravatar image tinadarko  ( 2014-12-02 10:23:51 -0600 )edit

If you use the min and max flow fields in the oa controller, OS should respect them. You might double check your idf to make sure you get what you intended. If you hard size the oa controller then your design oa spec at the zone will be used for sizing purposes, but the hard size values will be used during the operation. When validating I suggest that you look at air flow rates at standard density. If you want to turn off ventilation at night time then set the "Maximum Fraction of Outdoor Air Schedule Name" in the oa controller to 0 during unoccupied hours.

Kyle Benne gravatar image Kyle Benne  ( 2014-12-02 10:46:50 -0600 )edit

Will setting the "Minimum Outdoor Air Schedule" to 0 during unoccupied hours and using a hard sized OA flow rate be the same effect as setting the "Maximum fraction of outdoor air schedule" to 0 during unoccupied hours?

tinadarko gravatar image tinadarko  ( 2014-12-02 10:56:20 -0600 )edit

I believe so, but I have not personally tested this scenario in an OpenStudio generated idf. One thing we do is always write the mechanical ventilation controller which is referenced by the oa controller. This might complicate things if you go off script. Outside of OS you will often only see the mechanical vent controller used if you are doing demand controlled ventilation.

Kyle Benne gravatar image Kyle Benne  ( 2014-12-02 12:11:53 -0600 )edit

I just did a quick test run and saw that even with the min OA sch set to 0 during unoccupied hours the zone mech ventilation standard density flow rate does not entirely drop to 0 (screenshot link below). link text

Any thought on this?

Thanks!

tinadarko gravatar image tinadarko  ( 2014-12-02 12:47:24 -0600 )edit

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Asked: 2014-12-02 09:41:39 -0600

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Last updated: Jan 09 '15